The present invention relates to an ink jet recording head wherein a vibration plate, on the surface of which piezoelectric elements are formed, constitutes one part of a pressure generating chamber that communicates with nozzle orifices from which ink droplets are ejected when the piezoelectric elements are displaced, and relates as well to an ink jet recording apparatus incorporating the recording head.
In two types of ink jet recording heads that are now in practical use, vibration plates form parts of pressure generating chambers that communicate with nozzle orifices. When such a vibration plate is distorted in response to the vibration of a piezoelectric element, pressure is applied to ink in a pressure generating chamber and a drop of ink is ejected through an associated nozzle orifice. The head types for which vibration plates are used are: an ink jet recording head that employs piezoelectric actuators, in a vertical vibration mode, that are extended or compressed in the axial direction of the piezoelectric elements; and an ink jet recording head that employs piezoelectric actuators in a flexure vibration mode.
In the first head type, which is appropriate for high-resolution printing, the volumes of pressure generating chambers are changed by bringing end faces of piezoelectric elements into contact with a vibration plate. However, to manufacture this first type, a number of difficult procedures must be preformed, including the cutting of piezoelectric material to provide a comb-tooth-shaped device, which corresponds to the pitches of a nozzle orifice array, and the positioning and securing of the obtained piezoelectric element relative to the pressure generating chambers. Accordingly, the manufacturing process is complicated.
As for the second ink jet recording type, only a comparatively simple process, which involves the attachment of a piezoelectric green sheet having a shape that corresponds to the position of pressure generating chambers and the annealing of the resultant structure, is required to provide piezoelectric elements for a vibration plate. However, since flexure vibration is employed, specific areas are to a degree required, and providing for a high-resolution array is difficult.
To resolve the shortcomings of the second ink jet recording head, as proposed in Japanese Patent Publication No. 5-286131A, a thin-film formation technique is used to deposit a uniform piezoelectric layer across the entire surface of the vibration plate. A lithographic method is then used to subdivide the piezoelectric layer to provide shapes that match those of the pressure generating chambers and to form independent piezoelectric elements for the individual pressure generating chambers.
According to this method, the process by which piezoelectric elements are adhered to a vibration plate is not required, and the piezoelectric elements themselves can be formed using a precise and easily employed procedure, such as lithography. Further, since the piezoelectric elements that are produced in this way are thin, they are suitable for high-speed driving. In this case, while the piezoelectric layer that is deposited covers the entire surface of a vibration plate, only the upper electrode is formed for each pressure generating chamber. With this arrangement, the piezoelectric elements corresponding to the individual pressure generating chambers can be driven.
However, according to the manufacturing method for which the thin-film formation technique and the lithographic process are used, generally, the piezoelectric layer is formed so that it is thicker than the vibration plate in order to improve its piezoelectric characteristics. Therefore, when the piezoelectric elements are driven, the deforming efficiency is reduced by a neutral plane that is located inside the piezoelectric layer. As a result, the force of the displacement produced by the piezoelectric layer can not satisfactorily be converted into a force that can be used to eject ink.
To resolve this problem, it is one objective of the invention to provide an ink jet recording head that can more efficiently employ the force of the displacement developed when piezoelectric elements are driven, and an ink jet recording apparatus incorporating the recording head.
In order to achieve the above object, according to a first aspect of the present invention, there is provided an ink jet recording head comprising:
a pressure generating chamber communicating with a nozzle orifice from which ink is to be ejected;
a vibration plate constituting at least a part of the pressure generating chamber; and
a piezoelectric element including:
a lower electrode constituting at least a part of the vibration plate;
a piezoelectric layer formed on the lower electrode; and
an upper electrode formed on the piezoelectric layer, wherein an active part of the piezoelectric element for actuating the vibration plate to eject ink from the nozzle orifice is situated so as to face the pressure generating chamber, and wherein a neutral plane of the actuation of the piezoelectric element is located in the vibration plate.
Here, the vibration plate may be made of a ductile material. The vibration plate and the lower electrode may be made of the same material.
According to the first aspect, since the neutral plane is not located in the piezoelectric layer, only the force produced by compression is exerted by the piezoelectric layer that is driven, and the deforming efficiency provided by the piezoelectric elements is increased.
According to a second aspect of the invention, in the ink jet recording head of the first aspect, the following relationship is satisfied.
EsD2 greater than Efd2
where
Ef: Young""s modulus for the piezoelectric film and the upper electrode film
Es: Young""s modulus for the vibration plate
d: total thickness of the piezoelectric film and the upper electrode
D: thickness of the vibration plate
Preferably, the above relationship is satisfied in the active part of the piezoelectric element.
According to the second aspect, since the neutral plane is precisely positioned in the vibration plate, the deforming efficiency is improved.
According to a third aspect of the invention, in the ink jet recording head of the second aspect, EsD2 is 1 to 50 times as large as Efd2.
According to the third aspect, since the above relationship is defined within a predetermined range, the deforming efficiency is even more improved.
According to a fourth aspect of the invention, in the ink jet recording head of the first to the third aspects, the tensile stress of the lower electrode is greater than the stress of the piezoelectric layer.
According to the fourth aspect, the displacement of the piezoelectric layer due to the stress of the lower electrode is protected from being hindered.
According to a fifth aspect of the invention, in the ink jet recording head of the fourth aspect, the stress of the piezoelectric layer is tensile stress, and the tensile stress of the lower electrode is one to three times the tensile stress of the piezoelectric layer.
According to the fifth aspect, the displacement of the piezoelectric layer due to the stress of the lower electrode is more precisely protected from being hindered.
According to a sixth aspect of the invention, in the ink jet recording head of the first to the fifth aspects, the tensile stress of the upper electrode is greater than the stress of the piezoelectric layer.
According to the sixth aspect, the displacement of the piezoelectric layer due to the stress of the upper electrode is protected from being hindered.
According to a seventh aspect of the invention, in the ink jet recording head of the sixth aspect, the stress of the piezoelectric layer is tensile stress, and the tensile stress of the upper electrode is one to three times as large as the tensile stress of the piezoelectric layer.
According to the seventh aspect, the displacement of the piezoelectric layer due to the stress of the upper electrode is more precisely protected from being hindered.
According to an eighth aspect of the invention, in the ink jet recording head of the first to the seventh aspects, the thickness of the vibration plate is thicker than at least a total thickness of the active part of the piezoelectric film and the upper electrode.
The ink jet recording head may further comprise an insulating layer formed on the piezoelectric element. Here, the thickness of the vibration plate is thicker than a total thickness of the active part of the piezoelectric film, the upper electrode and the insulating layer. Preferably, the above relationship is satisfied in the active part of the piezoelectric element.
According to the eighth aspect, the positioning of the neutral plane in the vibration plate is ensured, and the deforming efficiency is improved.
According to a ninth aspect of the invention, in the ink jet recording head according to the first to the eighth aspects, the vibration plate includes, at the least, either a metal oxide film or a brittle film, and due to the driving of the piezoelectric member active unit, the neutral plane of the actuation of the piezoelectric element is located in the metal oxide film or in the brittle material film.
According to the ninth aspect, the stress that is imposed on the vibration plate is suppressed, and damage to or deterioration of the vibration plate can be prevented.
According to a tenth aspect of the invention, in the ink jet recording head of the ninth aspect, the brittle material film is made of zirconium oxide.
According to the tenth aspect, since the brittle film wherein the neutral plane is located is formed of a specific material, its destruction due to stress during displacement can be prevented.
According to an eleventh aspect of the invention, in the ink jet recording head of the ninth aspect, the metal oxide film is made of silicon oxide.
According to the eleventh aspect, since the metal oxide film wherein the neutral plane is located is formed of a specific material, its destruction due to stress during displacement can be prevented.
According to a twelfth aspect of the invention, in the ink jet recording head of the first to the eleventh aspects, the pressure generating chamber is formed in a monocrystalline silicon substrate by anisotropic etching, and the laminated structure of the piezoelectric element is formed by using a film formation technique and a lithographic process.
According to the twelfth aspect, a large number of ink jet recording heads having high resolution arrangements of nozzle orifices can be manufactured comparatively easily.
According to a thirteenth aspect of the invention, there is provided an ink jet recording apparatus comprising ink jet recording heads defined in the first to the twelfth aspects.
According to the thirteenth aspect, an ink jet recording apparatus having a greater number of reliable heads can be provided.